Exploration and appraisal drilling for hydrocarbons, encounter a number of hazards which dictate the planning of the drilling operation and the well construction. One of these hazards is the presence of formation fluids or gases, which have a pressure anomaly known generally as “abnormal pore pressure”, consisting of a difference of pore pressure to that which would be predicted from depth and density of that fluid or gas. Where the pressure is greater than expected the abnormal pore pressure is known as overpressure, and where it is less than expected the abnormal pore pressure is known as depleted pressure. It is important to predict the occurrence of such hazards ahead of time to allow for appropriate drilling fluid design, and to provide constraints on the design of the borehole size and the type and size of casing. If inadequately predicted, the effects of overpressure can extend from costly delays in drilling, non-completion of well objectives and in the extreme a loss of well through blow-out or engineering failure. Effects of depleted pressure may include loss of drilling tools in the borehole and fracture pressure of exposed Earth formations that is too low to safely remain unprotected while drilling proceeds through other Earth formations having higher fluid pressures.
Overpressure can arise from a number of causes though primarily from a restriction in the flow of pore fluid from a fluid filled reservoir during the reservoir burial history. Depleted pressures typically result from extraction of fluids from sub-surface fluid reservoirs.
The difference between total pressure or stress applied to a reservoir and pore pressure within the reservoir is called the effective stress or differential pressure, and this is related to the amount of abnormal pore pressure encountered. Low effective stress in general corresponds to areas of high abnormal pore pressure (ie overpressure) while large effective stress corresponds to areas of low overpressure.
Current methods for quantitative prediction of overpressure are derived from the relationship between velocity and effective stress as determined from empirical evaluation of in-situ stresses and velocities.
However there is uncertainty in predicting overpressure using this method which is related to the quality and source of the velocity data. If surface seismic data is used then there are levels of sophistication of processing in assessing the velocities but these are relatively expensive and computer intensive. If the source is well data there is an increased reliability of the velocity data, but its predictive value is limited since one has to drill the well in order to derive the velocity data. This information can be obtained from wells offset from a proposed drilling location.
Throughout this specification the term “comprising” is used inclusively, in the sense that there may be other features and/or steps included in the invention not expressly defined or comprehended in the features or steps subsequently defined or described. What such other features and/or steps may include will be apparent from the specification read as a whole.
Throughout this specification, except where the context required otherwise due to express language or necessary implication, the expression “seismic data” or variations such as “Seismic trace” or “seismic waveform” is used to denote data, traces or waves derived from seismic surveying including surface seismic and VSP surveying as well as wireline derived or logged data, traces or waveforms.